The present disclosure relates generally to electronic memory technology, and more specifically to the introduction of a hydrogenated amorphous semiconductor material encapsulation layer to a memory cell to improve its coercivity and other performance characteristics.
Magnetoresistive random access memory (MRAM) is a non-volatile memory that combines a magnetic device with standard silicon-based microelectronics to obtain the combined attributes of non-volatility, high-speed read/write operations, high read/write endurance and data retention. Data is stored in MRAM as magnetic states or characteristics (e.g., polarity or magnetic moment) instead of electric charges. In a typical configuration, each MRAM cell includes a transistor, a magnetic tunnel junction (MTJ) device (i.e., memory cell) for data storage, a bit line and a word line. In general, the MTJ's electrical resistance will be high or low based on the relative magnetic states of certain MTJ layers. Data is written to the MTJ by applying certain magnetic fields or charge currents to switch the magnetic states of the MTJ layers. Data is read by detecting the resistance of the MTJ. Using a magnetic state/characteristic for storage has two main benefits. First, unlike electric charge, magnetic state does not leak away with time, so the stored data remains even when system power is turned off. Second, switching magnetic states has no known wear-out mechanism.
The term “magnetoresistance” describes the effect whereby a change to certain magnetic states of the MTJ storage element results in a change to the MTJ resistance, hence the name “Magnetoresistive” RAM. A typical MTJ structure includes a stacked configuration having a fixed magnetic layer (e.g., Fe, CoFe, CoFeB, etc.), a thin dielectric tunnel barrier (e.g., MgO) and a free magnetic layer (e.g., Fe, CoFe, CoFeB, etc.). The MTJ has a low resistance when the magnetic moment of its free layer is parallel to the magnetic moment of its fixed layer. Conversely, the MTJ has a high resistance when its free layer magnetic moment is oriented anti-parallel to its fixed layer magnetic moment. The MTJ can be read by activating its associated word line transistor, which switches current from a bit line through the MTJ. The MTJ resistance can be determined from the sensed current, which is itself based on the polarity of the free layer. Conventionally, if the fixed layer and free layer have the same polarity, the resistance is low and a “0” is read/written. If the fixed layer and free layer have opposite polarity, the resistance is higher and a “1” is read/written.
A practical MRAM or STT-MRAM device integrates a plurality of magnetic memory elements with other circuits such as, for example, control circuits for the magnetic memory elements, comparators for detecting the states in the magnetic memory elements, input/output circuits and miscellaneous support circuitry. As a result, a variety of microfabrication processing challenges must be overcome before high capacity/density MRAM products become commercially available. For example, CMOS technology is typically required in order to reduce the power consumption of the device and provide a variety of support functions. As is known in the art, various CMOS processing steps (such as annealing implants) are carried out at temperatures in excess of 300 Celsius. On the other hand, ferromagnetic materials employed in the fabrication of MRAM devices, such as CoFe and NiFeCo for example, require substantially lower process temperatures in order to prevent intermixing of magnetic materials. Thus, the magnetic memory elements are designed to be integrated into the back-end-of-line (BEOL) wiring structure following front-end-of-line (FEOL) CMOS processing.
MTJs contain component layers that are easily oxidized and also sensitive to corrosion. To protect MTJ memory cells from BEOL fabrication steps and performance degradation over time, as well as maintain post-fabrication performance and reliability of the MRAM device, it is desirable to form during fabrication an encapsulation layer over the memory cell. Silicon nitride and similar compounds are desirable as encapsulation materials for their adhesion to MTJ metal surfaces, and for their strong interfacial bonds that inhibit migration of metal atoms along the dielectric/metal interfaces of the MTJ. Such metal migration is a known cause of MTJ thermal degradation, and can limit processing temperatures in patterned MTJ memory cells to below 300° C.
To prevent degradation of the MTJ memory cell, BEOL thermal budget for MRAM devices (e.g., <about 250 Celsius to about 300 Celsius) is significantly lower than for conventional semiconductor fabrication processes (˜400 Celsius). This can affect the intrinsic quality of dielectrics being used in the BEOL, and can worsen seams and void formations around the topographical features that are being encapsulated. Low thermal budget also prevents the use of certain post-processing passivation anneals, and packaging materials and processes.
In addition to fabrication techniques that protect the memory cell during fabrication, maintain post-fabrication performance and maintain reliability, it would be beneficial to provide fabrication techniques and resulting device structures and characteristics that improve post-fabrication performance and reliability. For example, stability of the MTJ device is increased by improving its coercivity (HO. Coercivity is the intensity of the applied magnetic field required to reduce the magnetization of that material to zero after the magnetization of the sample has been driven to saturation. Thus, improving coercivity improves the resistance of a ferromagnetic material to becoming demagnetized. It would be further beneficial to provide such fabrication techniques and resulting device structures and characteristics without significantly increasing the BEOL thermal budget.